a. Field of the Invention
The present invention relates to an optical system for an endoscope and, more particularly, to an optical system for an endoscope for which the optical axis of the objective is slightly shifted in parallel with the center line of the image fiber.
B. Description of the prior art
As known widely, an endoscope is used for observing the inner wall etc. of a body cavity such as a stomachs by inserting into the body cavity. Besides, as it is necessary to cut off the affected part by using a forceps means while observing by means of the endoscope, endoscopes are generally provided with forceps means for cutting off the affected part. As, however, the position of said forceps means is considerably distant from the optical axis of the optical system of the endoscope, known endoscopes have a disadvantage that it is difficult to observe the position of the forceps means when cutting the affected part by the forceps means while observing the affected part. To eliminate the above-mentioned disadvantage, known optical systems for endoscopes are provided with a prism or reflecting mirrors in front of the objective as shown in FIG. 1 and FIG. 2. In the example shown in FIG. 1, a prism 3 is provided in front of the optical system for endoscopes which comprises an image fiber 1 for transmitting the image of the object and objective 2. By utilizing refraction of said prism 3, the forceps means is put to a position near the center of the field of the optical system. In the example shown in FIG. 2, a prism 4 is provided in front of the objective 2. By means of reflection by reflecting surfaces 4a and 4b of said prism 4, the forceps means is put to a position near the center of the field. (In the latter case, it is of course all right to provide two reflecting mirrors, which are combined to each other, instead of said prism 4.)
The above-mentioned known means, however, have the following disadvantages. In case of the former example shown in FIG. 1, astigmatism is caused and, moreover, shifting of the image for different wavelengths is caused by dispersion of the prism. Furthermore, when it is attempted to make the field angle of the optical system wide, it is necessary to make the prism 3 large and, therefore, the distal end of the endoscope becomes also large when such large prism is provided in the distal end. In case of the latter example shown in FIG. 2, two reflecting surfaces exist and the optical axis is deflected by said reflecting surfaces. Consequently, the distal end becomes large and, moreover, flare tends to occur.